It is the primary purpose of this project to study some of the pertinent factors which influence cell differentiation and malignant transformation, using techniques and approaches which range from the microscopic to the molecular level. Particular emphasis is given to those systems in which murine RNA tumor viruses or their precursors may be the transforming agent. A variety of mouse model systems are used, including plasma cell tumors, mammary tumors, and neuroblastomas. Current projects include: 1) characterization of intracisternal A particles (IAP) with particular emphasis on homology to known RNA tumor viruses, molecular events associated with replication, conservation and divergence of IAP genes, and factors controlling the expression of IAP genes; 2) effects of interferon on the assembly and maturation of murine oncornaviruses with special emphasis on the study of mechanisms whereby whole virions are rendered noninfectious.